The invention concerns a process for determining the speed of a direct current motor and a circuit arrangement for carrying out this process.
It is known that direct current motors are used for many applications where particular importance is assigned to the actual speed or a speed regulation of the direct current motor. Thus, direct current motors are used, for example, as adjustment motors in motor vehicles, e.g. for a seat adjustment, for opening windows, for sliding roofs and sun roofs, for ventilators, fans, windshield wipers etc. In order to determine the speed of direct current motors, it is known to assign a speed sensor to these, by means of which the rotational movement of an armature shaft of the direct current motor is detected. A measuring signal that is proportional to the speed and is separated by the speed sensor can then be processed further. The DE-OS 42 16 040, for example, shows that it is known to use a measuring signal, detected in this way and proportional to the speed, for monitoring a direct current motor for thermal overload. However, it is a disadvantage in this case that the arrangement of the additional speed sensor complicates the design of the direct current motor.
In the electrical engineering field in general, it is known that the rotational speed of a direct current motor is proportional to the source voltage, which is induced in its armature during the operation of the direct current motor. An error occurs during the transition from induced source voltage/direct current motor speed as a result of a temperature dependence of the magnetic flow. Further momentary operating states of the direct current motor, which actually occur, such as a fluctuating supply voltage, a changing load moment, lead to a distortion of the determination of the induced source voltage. On the whole, this leads to an incorrect determination of the actual speed for the direct current motor.
The publication Elektronik 1, Jan. 13, 1994, pages 40 to 41, shows that it is known to regulate the speed of a direct current motor. The actual speed of the direct current motor, in this case, is measured by way of a generator voltage that is proportional to the speed, that is to say without a speed sensor.